1. Field of the Invention
The present invention relates to an improved process for the purification of crude 1,1-dichloro-1-fluoroethane by treatment with chlorine followed by distillation.
2. Description of the Related Art
1,1-Dichloro-1-fluoroethane (HFA-141b) is a partially halogenated chlorofluorinated hydrocarbon which is proving to be an advantageous substitute for certain entirely halogenated chlorofluorinated hydrocarbons (CFCs) whose production and use are being progressively reduced because they are suspected of having a harmful effect on the ozone layer.
Crude 1,1-dichloro-1-fluoroethane from the synthesis processes is generally contaminated by by-products of the synthesis and undesirable impurities. Certain of these by-products and impurities can be easily separated by distillation. This is especially the case for 1-chloro-1,1-difluoroethane, 1,1,1-trifluoroethane or 1,1,1-trichloroethane, as well as for the heavier compounds comprising a greater number of carbon atoms, formed during the reaction. However, 1,1-dichloro-1-fluoroethane generally contains as impurities small amounts of chlorinated and/or chlorofluorinated unsaturated compounds whose separation by distillation proves to be difficult, in view of their boiling point in the neighborhood of that of 1,1-dichloro-1-fluoroethane.
Besides vinylidene chloride, which is the most important impurity, the unsaturated impurities which may be present in 1,1-dichloro-1-fluoroethane to be purified are mainly cis- and trans-1,2-dichlorofluoroethylenes, trans-1,2-dichloroethylene, dichloroacetylene and 1-chloro-1-fluoroethylene.
Patent Application EP-A-0,401,493 from Atochem North America describes the purification of 1,1-dichloro -1-fluoroethane by photochemical chlorination of the unsaturated impurities followed by a separation by distillation. This process is implemented at a temperature such that 1,1-dichloro-1-fluoroethane is in the liquid phase at the operating pressure. The operating conditions are also in part limited by the presence of the light energy source.
The accent is put in this document on the purification from vinylidene chloride but, in the majority of the examples, it was not possible to fall below 120 ppm by weight of this compound. Additionally, the removal of cis- and trans-1,2-dichlorofluoroethylenes was not mentioned, which are more halogenated impurities whose conversion appears more difficult as, in certain cases, their concentration can increase again after having passed through a minimum. By way of reference, let it be pointed out that the PAFT II specifications (Program for Alternative Fluorocarbon Toxicity testing) impose a total chlorofluorinated unsaturated impurities content not exceeding 10 ppm by weight.
The present invention consequently has the aim of providing an improved process for the purification of crude 1,1-dichloro-1-fluoroethane by treatment with chlorine followed by a distillation which not only makes possible a better purification from vinylidene chloride but also an effective and rapid separation from the other unsaturated impurities, especially cis- and trans-1,2-dichlorofluoroethylenes.